Process for the stereoselective synthesis of 3-substituted 2-thiomethylpropionic acids

ABSTRACT

Process for the stereoselective synthesis of 3-substituted 2-thiomethylpropionic acids 
     Compounds of the formula I ##STR1## in which R 1  and R 2  are as defined can be prepared stereoselectively by reaction of acrylic acid or derivatives thereof or propionic acid or derivatives thereof with a chiral auxiliary, reaction with a mercaptan, stereoselective alkylation and subsequent hydrolysis and oxidation.

This application is a 371 of PCT/EP93/02673 filed Sep. 30, 1993.

3-substituted 2-thiomethylpropionic acids and derivatives thereof, for example esters, are gaining interest as building blocks and as precursors for aspartyl protease inhibitors [P. Buhlmeyer et al., J. Med. Chem. 31 (1988), 1839; R. Henning, Nachr. Chem. Techn. Lab. 38 (1990), 460; J. R. Huff, J. Med. Chem. 34 (1991), 2305].

The compounds mentioned are already known. Moreover, a series of syntheses or processes for the preparation of these compounds has been described.

In this context, examples of some recent work include:

P. Buhlmeyer et al., J. Med. Chem. 31 (1988), 1839; K. Tsuji et al., Tetrahedron Lett. 30 (1989), 6189; M. Nakano et al., Tetrahedron Lett. 31 (1990), 1569; M. Nakano et al., Chem. Lett. (1990), 505; cf. also D. A. Evans et al., J. Org. Chem. 50 (1985), 1830.

The first-mentioned synthesis requires, inter alia, the use of formaldehyde and/or formaldehyde derivatives, which, like the by-products formed in this case, are regarded as being highly objectionable in terms of health because of their alkylating properties and necessitate specific safety measures which include the area of workplace health and safety [Merck Index 11, 4150].

Moreover, the preparation of optically pure compounds requires the splitting of a racemate. For this purpose the racemic acid is converted into the diastereomeric amides using L-phenylalaninol, these amides are separated by chromatography, and the desired isomer is obtained by hydrolysis of the corresponding amide. The large number of steps, the fundamental disadvantages of splitting a racemate and the scale-up problems involved in a chromatographic purification mean that this route appears unattractive for the preparation of relatively large quantities.

The synthesis according to Tsuyi et al. leads to optically pure material with a similar number of steps but without splitting a racemate. This synthesis comprises a number of steps for the introduction and elimination of protecting groups; some of the reagents employed are expensive, and in some cases carcinogenic formaldehyde derivatives are used or produced in the course of the syntheses [H. G. Neumann in "Allgemeine und spezielle Pharmakologie und Toxikologie" [General and Special Pharmacology and Toxicology], 4th edition, W. Forth, ed., B. I. Wissenschaftsverlag, Mannheim-Vienna-Zurich, p. 621 ff. (1983); Arch. Environ. Health 30 (2), 61]. Therefore, this synthesis route does not constitute an economically and ecologically justifiable alternative for the preparation of large quantities.

The preparation of 2-mercaptomethyldihydrocinnamic acid, which is shown in the context of the synthesis of thiorphan described by Evans and Mathre, is short in comparison with the two syntheses which have already been discussed, proceeds with a good overall yield, and enables the specific preparation of both enantiomers in high optical purity. The weak point in this synthesis is the complex introduction of the mercaptan grouping and the consequent necessity to use benzylthiomethyl bromide, which is objectionable in terms of health and is prepared from trioxane (as formaldehyde source), benzyl mercaptan (or benzylthiomethyl chloride) and HBr [H. G. Neumann in "Allgemeine und spezielle Pharmakologie und Toxikologie" [General and Special Pharmacology and Toxicology], 4th edition, W. Forth, ed., B. I. Wissenschaftsverlag, Mannheim-Vienna-Zurich, p. 621 ff. (1983); Arch. Environ. Health 30 (2), 61].

Nakano et al. describe two routes for the synthesis of the carbon framework of the compounds I. The first synthesis starts from diethyl malonate, and requires six steps and, in addition, chromatographic resolution of the diastereomers.

The second route proceeds via a chiral, non-racemic arylpropionyloxazolidinone and its stereoselective alkylation using benzyl bromomethyl ether [M. W. Holladay et al., J. Med. Chem. 30 (1987), 374] followed by chromatography. After removal of the benzyl group by hydrogenation, as in the case of the first route, the sulfur substituent is introduced by tosylation of the free hydroxyl group followed by substitution by NaSCH₂ CH₃ in DMF.

These two synthesis routes are also characterized by: a high number of steps, carcinogenic and toxic formaldehyde derivatives as precursors or reagents, the complex introduction of the sulfur substituent, which is a process which is not entirely free from racemization, and, last but not least, the chromatographic separation of impurities and resolution of diastereomeric compounds.

The object on which the present invention is based is to develop a process for the synthesis of compounds of the formula I which

has a low number of steps,

does not require protecting group chemistry,

does not require column chromatography steps,

is stereoselective and leads selectively to the enantiomeric compounds of the formula I,

does not require splitting of racemates or resolution of diastereomers, and

represents an improvement, or is unobjectionable, from the points of view of health, ecology and safety.

This object is achieved by the process according to the invention. The subject of the invention is consequently a process for the stereoselective preparation of a compound of the formula I ##STR2## in which R¹ is C₁ -C₆ -alkyl, C₃ -C₆ -cycloalkyl, (C₆ -C₁₂)-aryl-(C₁ -C₄)-alkyl or C₆ -C₁₂ -aryl or -heteroaryl or -heterocycloalkyl which may be substituted by 1, 2 or 3 identical or different hydroxyl, methoxy or trialkylsilyloxy groups; and

R² is C₆ -C₁₂ -aryl which may be substituted by 1, 2 or 3 identical or different methoxy, halogen, cyano, methyl, trifluoromethyl, isopropyl or nitro groups;

is C₃ -C₉ -heteroaryl which may be substituted by 1, 2 or 3 identical or different methoxy, halogen, cyano, methyl, trifluoromethyl, isopropyl or nitro groups; or

is C₁ -C₁₀ -alkyl, alkenyl or alkynyl which may be substituted by 1, 2 or 3 identical or different methoxy, halogen, cyano, methyl, trifluoromethyl, isopropyl or nitro groups

and the compounds of the formula I may be present in the R or S form, which comprises converting a compound of the formula IIa or IIb or IIIa or IIIb, respectively, (chiral auxiliary), in which Y is =O or S, ##STR3##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.5 =C.sub.6 -C.sub.10 aryl, R.sup.4 =R.sup.6 =H,                                              IIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.10 aryl, R.sup.3 =R.sup.5 =H,                                              IIb

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      IIIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,                      IIIb

a₁) with acrylic acid or with derivatives of acrylic acid, into a compound of the formula IVa or IVb or Va or Vb, respectively, in which Y is =O or S, ##STR4##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.5 =C.sub.6 -C.sub.10 -aryl, R.sup.4 =R.sup.6 =H,                                              IVa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.10 -aryl, R.sup.3 =R.sup.5 =H,                                              IVb

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      Va

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.2 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,                      Vb

or

a₂) with propionic acid or propionic acid derivatives which are in turn substituted in position C₃ by a radical X=Cl, Br, OTs, OMs, into a compound of the formula VIa or VIb or VIIa or VIIb, respectively, in which X and Y are as defined above, ##STR5##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.5 =C.sub.6 -C.sub.10 -aryl, R.sup.4 =R.sup.6 =H,                                              VIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.10 -aryl, R.sup.3 =R.sup.5 =H,                                              VIb

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      VIIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl- C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,            VIIb

b) reacting the compound of the formula IVa, IVb, Va, Vb, VIa, VIb, VIIa or VIIb obtained according to a₁) or a₂), directly or after isolation in a nonaqueous medium or in the presence of phase-transfer catalysts in aqueous two-phase systems, with a mercaptan of the formula R¹ SH, in which R¹ is as defined above, into the corresponding compound of the formula VIIIa or VIIIb or IXa or IXb, respectively, in which R¹ and Y are as defined above, ##STR6##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.5 =C.sub.6 -C.sub.10 -aryl, R.sup.4 =R.sup.6 =H,                                              VIIIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.10 -aryl, R.sup.3 =R.sup.5 =H,                                              VIIIb

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      IXa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,

or

a₃) converting a compound of the formula IIa or IIb or IIIa or IIIb, respectively, in the presence of a quantity of base which is sufficient for the deprotonation of the oxazolidinone and of the mercaptan R¹ SH, in which R¹ is as defined above, into a compound of the formula VIIIa or VIIIb or IXa or IXb, respectively, by reaction with acryloyl or propionyl halides or their respective mixed anhydrides and further reaction of the reaction product with the mercaptan in one step,

c) then converting the compound obtained according to b) or a₃) into the corresponding compound of the formula Xa, Xb, XIa or XIb by stereoselective alkylation, ##STR7##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.4 =R.sup.6= H, R.sup.5 =C.sub.6 -C.sub.12 -aryl,                                          Xa

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      XIa ##STR8##

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6= C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =H,                                              Xb

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,                      XIb

in which Y, R¹ and R² are as defined above,

d) then converting the compound obtained in this way by reaction with H₂ O₂ in alkaline solution, preferably with LiOH/H₂ O₂, into compounds of the formula XII which may be present in the R or S form, ##STR9## and

e) then converting the compound obtained in this way, by further oxidation, into the corresponding compound of the formula I.

The process according to the invention is particularly suitable for the preparation of compounds of the formula I in which

R¹ is C₁ -C₄ -alkyl, C₅ -C₆ -cycloalkyl, (C₆ -C₁₂)-aryl-(C₁ -C₃)-alkyl or C₆ -C₁₂ -aryl or -heteroaryl or -heterocycloalkyl which may be substituted by a hydroxyl, methoxy or trialkylsilyloxy group;

R² is C₆ -C₁₂ -aryl which may be substituted by a methoxy, halogen, methyl, trifluoromethyl or isopropyl group;

is C₃ -C₆ -heteroaryl which may be substituted by a methoxy, halogen, methyl, trifluoromethyl or isopropyl group, or

is C₁ -C₆ -alkyl, -alkenyl or -alkynyl.

Furthermore, the process according to the invention is particularly suitable for the preparation of compounds of the formula I in which

R¹ is C₁ -C₄ -alkyl, (C₆ -C₁₂)-aryl-(C₁ -C₃)-alkyl or C₆ -C₁₂ -aryl, and

R² is =C₆ -C₁₂ -aryl, C₃ -C₆ -heteroaryl or C₁ -C₄ -alkyl, -alkenyl or -alkynyl.

The process according to the invention has very special importance for the preparation of compounds of the formula I in which R¹ is =tert-butyl and R² is =naphthyl.

The abovementioned substituent C₆ -C₁₂ -aryl is understood to be, for example, phenyl, naphthyl or biphenyl.

Alkyl, alkenyl and alkynyl may be straight-chain or branched.

Halogen is preferably fluorine, chlorine or bromine, particularly preferably chlorine.

Heterocycloalkyl or, respectively, heteroaryl comprises in particular compounds which contain up to 3, preferably and in particular 1 N, S and/or O atom in the ring. Examples which may be mentioned are pyridyl, furyl, pyrimidyl, pyrrolidino, pyrolyl, piperidino or piperidyl, especially pyridyl.

Compounds of the formulae IIa, IIb, IIIa and IIIb which are preferably used are those in which,

    R.sup.3 =Me, R.sup.5 =Ph, R.sup.4 =R.sup.6 =H              IIa

    R.sup.4 =Me, R.sup.6 =Ph, R.sup.3 =R.sup.5 =H              IIb

    R.sup.3 =CH(CH.sub.3).sub.2, R.sup.4 =R.sup.5 =R.sup.6 =H  IIIa

    R.sup.4 =CH(CH.sub.3).sub.2, R.sup.3 =R.sup.5 =R.sup.6 =H  IIIb

and Y is =O.

Important intermediates in these synthesis routes are:

(4R,5S)- and (4S,5R)-4-methyl-5-phenyl-2-oxazolidinone, (S)- and (R)-4-isopropyl-2-oxazolidinone

(4R,5S)- and (4S, 5R) -3-acryloyl-4-methyl-5-phenyl-2-oxazolidinone,

(4S)- and (4R)-3-acryloyl-4-isopropyl-2-oxazolidinone,

(4R,5S)-3- and (4S,5R)-3-[1-oxo-3-halopropyl]-4-methyl-5-phenyl-2-oxazolidinone,

(4R,5S)-3- and (4S,5R)-3-[1-oxo-3-(alkyl- or arylsulfonyloxy)propyl]-4-methyl-5-phenyl-2-oxazolidinone,

(4S)- and (4R)-3-[1-oxo-3-halopropyl]-4-isopropyl-2-oxazolidinone,

(4S)- and (4R)-3-[1-oxo-3-(alkyl- or arylsulfonyloxy)propyl]-4-isopropyl-2-oxazolidinone,

(4R,5S)-3- and (4S,5R)-3-[1-oxo-2-(alkyl- or arylthio)methyl]propyl-4-methyl-5-phenyl-2-oxazolidinone,

(4S) and (4R)-3-[1-oxo-2-(alkyl- or arylthio)methyl]propyl-4-isopropyl-2-oxazolidinone,

(4R,5S)-3-[(2S)-1-oxo-2-(alkyl- or arylthiomethyl)-3-(alkyl- or aryl)propyl]-4-methyl-5-phenyl-2-oxazolidinone,

(4S,5R)-3-[(2R)-1-oxo-2-(alkyl- or arylthiomethyl)-3-(alkyl- or aryl)propyl]-4-methyl-5-phenyl-2-oxazolidinone

(4S)-3-[(2R)-1-oxo-2-(alkyl-or arylthiomethyl)-3-(alkyl- or aryl)propyl-4-isopropyl-2-oxazolidinone,

(4R)-3-[(S)-1-oxo-2-(alkyl- or arylthiomethyl)-3-(alkyl- or aryl)propyl]-4-isopropyl-2-oxazolidinone and

(2S)- and (2R)-2-(alkyl- or arylsulfoxymethyl)-3-(alkyl- or aryl)propionic acid.

The acryloyl compounds of the formulae Ira, IVb, Va or Vb and the propionyl compounds of the formulae VIa, rib, VIIa or VIIb can be obtained by complete, irreversible deprotonation of the compounds of the formulae IIa, IIb, IIIa or IIIb, which are readily accessible in good yields [D. A. Evans, D. J. Mathre, J. Org. Chem. 50 (1985), 1830; D. A. Evans, J. R. Gage, Org. Synth. 68 (1989), 77; P. G. M. Wuts, L. E. Pruitt, Synthesis 1989, 622], with sodium hydride, BuLi, KOtBu in anhydrous aprotic solvents such as tetrahydrofuran, dimethoxyethane, tert-butyl methyl ether, toluene, followed by reaction with the corresponding carboxylic acid halides, preferably the acid chlorides or the mixed anhydrides (prepared, for example, from acrylic or 3-chloro- or bromo-, propionic acid, triethylamine and carboxylic acid chlorides such as pivaloyl chloride) at low temperatures (-80° C. to -20° C.). It was possible to do without the stabilization with Cu/Cu₂ Cl₂ in the case of the acryloyl compounds (see Binger et al., Liebigs Ann. Chem. 1989, 739; Oppolzer et al. Tetrahedron Lett. (1991), 4893).

The preferred procedure, especially for the synthesis of the acryloyl compounds, is reaction of the oxazolidinones at room temperature in tetrahydrofuran with sodium hydride, followed by cooling to from -80° C. to -60° C. and reaction with the acid chloride.

In order to carry out the conjugate mercaptan addition, the appropriate thio compounds are reacted, in solution in ether (preferably THF), with alkali metal hydrides (preferably NaH) or appropriate organometallic compounds (preferably BuLi) or with fluorides (e.g. tetra-n-butylammonium fluoride) and are then reacted with the compounds of the formulae IVa, IVb, Va or Vb.

As an alternative to this, the mercaptan addition can also be carried out with phase-transfer catalysis (e.g. with methyltrioctylammonium chloride or tetra-n-butylammonium hydrogen sulfate) in a mixture of aqueous NaOH and organic solvents such as methylene chloride, dichloroethane, tetrahydrofuran, dimethoxyethane, tert-butyl methyl ether, diisopropyl ether, etc.

A further possibility for the synthesis of the compounds VIIIa,b, IXa,b is the reaction of compounds of the formula VIa,b, VIIa,b with mercaptans of the formula R¹ SH, in which R¹ is as defined above, in the presence of bases such as K₂ CO₃ in protic solvents such as ethanol.

However, because of the higher overall yield, the nature of the by-products and the increased ease and rapidity of working up and isolating the products, "one-pot" variants have proven advantageous. Such variants lead, in accordance with the invention, in one step directly from compounds of the formulae IIa or IIb or IIIa or IIIb, respectively, to compounds of the formulae VIIIa or VIIIb and/or IXa or IXb, respectively. For this purpose the chiral auxiliaries of the formulae IIa or IIb or IIIa or IIIb, respectively, are reacted with a quantity of base (NaH, BuLi, etc.) which is sufficient both for the deprotonation of the oxazolidinones and for the mercaptan, acryloyl halide or propionyl halide or their respective mixed anhydride is added after complete deprotonation, and the respective mercaptan is added to the resulting mixture after acylation is complete (monitoring by TLC).

Furthermore, the following procedure is preferred: After completion of the NaH- or BuLi-mediated acylation of the compounds of the formulae IIa, IIb, IIIa or IIIb with acryloyl or propionyl halide or with the respective mixed anhydrides at from -80° C. to -60° C., proton donors such as, for example, acetic acid, citric acid, water are added, the mixture is heated to room temperature, a pH from 9 to 12 is established (for example using aqueous NaOH) and the mercaptan and a phase-transfer catalyst such as, for example, tetra-n-butylammonium hydrogen sulfate, methyltrioctylammonium chloride are added.

The alkylation of the compounds of the formulae VIIIa, VIIIb, IXa or IXb is preferably carried out in anhydrous aprotic solvents, preferably in THF, by stereoselective enolate formation (D. A. Evans, Pure Appl. Chem. 53 (1981) 1109; D. A. Evans, Aldrichimica Acta 15 (1982), 23) for example at from -80° C. to -60° C. using lithiated secondary a mines (e.g. LDA) or Na or Li hexamethyldisilazane (NaHMDS, LiHMDS) followed by reaction with alkyl or aryl halides, preferably at from -50° C. to -10° C.

The subsequent mild elimination of the chiral auxiliary is carried out using H₂ O₂ in alkaline solution at from -10° to -30° C., preferably using LiOH/H₂ O₂ at from 0° C. to 25° C., preferably in solvents such as dioxane, THF, dimethoxyethane or tBuOMe or aqueous mixtures of these solvents (Evans et al., Tetrahedron Lett. 28 (1987), 6141).

If desired, the compounds of the formula Xa, XIa, Xb, XIb can be converted into carboxylic acid derivatives such as esters and amides, or into alcohols, by transesterification (for example with LiOR, Ti(OR)₄ or BrMgOR), by transamination (for example with Me₂ AlN(OR)R) or by reduction, for example with LiAlH₄, LiBH₄. In this context R is Me, Et, CH₂ Ph, isopropyl or n-butyl.

In the case of the LiOH/H₂ O₂ -induced hydrolysis of the compounds of the formulae Xa, Xb, XIa or XIb this leads, as planned, directly to the sulfoxides of the formula XII. The oxidation to give the compounds of the formulae I is possible both directly and also after prior isolation of the sulfoxides.

Suitable oxidizing agents are:

metachloroperbenzoic acid (mCPBA), for example in CH₂ Cl₂ at 0°-25° C.;

potassium permanganate (KMnO₄), for example in H₂ O/K₂ CO₃ at room temperature;

Oxone (2KHSO₅.KHSO₄.K₂ SO₄) in water at room temperature;

magnesium monoperoxyphthalate (MMPP) in water at room temperature;

particularly preferred is H₂ O₂, 30% strength, at room temperature in CH₂ Cl₂, in the presence of acetic acid and polyphophoric acid.

A great advantage in process terms, especially for large batches, results from the combination of alkylation, hydrolysis and oxidation into a one-pot process.

Following direct conversion of the compounds of the formulae VIIIa, VIIIb, IXa or IXb into compounds of the formulae XII or I, the separation of the unwanted byproducts and the removal of the excess reagent is limited to a simple extractive working-up of the basified reaction mixture (e.g. partition between water and methylene chloride). After acidification of the aqueous phase and extraction with ethyl acetate, the desired compounds of the formulae XII or I are obtained in high chemical and optical purity.

The process according to the invention therefore comprises a sequence of not more than 4 steps with yields of 60-95% (the overall yield is between 30 and 40%). The working up of the intermediates and end products does not require column chromatography but is limited to crystallization, extraction and filtration. The starting compounds used are inexpensive and represent no problems from the working and safety point of view.

    ______________________________________                                         List of abbreviations                                                          ______________________________________                                         Bu              butyl                                                          LDA             lithium diisopropylamide                                       Me              methyl                                                         OTs             p-toluenesulfonyl                                              OMs             methylsulfonyl                                                 Ph              phenyl                                                         t               tertiary                                                       THF             tetrahydrofuran                                                ______________________________________                                    

The examples which follow are intended to illustrate the invention in more detail:

EXAMPLE 1 Synthesis of Compound IVa Where R³ =methyl, R⁵ =Phenyl

1.8 g (45 mmol) of NaH (60% pure) are placed in 50 ml of THF, and 5 g (28 mmol) of the oxazolidinone of the formula IIa where R³ =Me, R⁵ =Ph in 20 ml of THF are added at from 20° to 25° C. The mixture is stirred for one hour at from 20° to 25° C. and then cooled to -70° C., and 4.7 ml (58 mmol) of acryloyl chloride are added dropwise. The temperature is maintained at between -60° and -65° C. After stirring for 10 minutes the reaction has ended. 50 ml of saturated NaHCO₃ solution are added dropwise. During this addition the temperature rises to about -10° C. The cooling bath is then removed, ethyl acetate is added, the phases are separated, the organic phase is washed twice with saturated NaCl solution and dried with Na₂ SO₄ and the solvent is distilled off in vacuo. The residue is taken up in tert-butyl methyl ether and the precipitate is separated off. The solution which remains (filtrate) contains 4.6 g (69%) of the desired product.

¹ H-NMR (200 MHz, CDCl₃): σ=0.95 (d; 3H, CHCHH₃); 4.82 (quint; 1H, CHCHCH₃); 5.70 (d; 1H, CHCHPh); 5.92 (dd; 1H, olef.); 6.57 (dd; 1H, olef.); 7.1-7.5 (m; 5H, Ph); 7.53 (dd; 1H, olef.).

EXAMPLE 2 Synthesis of Compound VIIIa Where R¹ =C(CH₃)₃, R³ =Methyl, R⁵ =Phenyl

10 ml of toluene, 5 ml of water, 1 ml of 5N NaOH, 20 mg of tetra-n-butylammonium hydrogen sulfate and 0.76 ml (6.74 mmol) of tert-butyl mercaptan are added to 1.39 g (6.0 mmol) of the compound IVa where R³ =Me, R⁵ =Ph, and the mixture is stirred at room temperature for 2 h. Following the addition of 100 ml of water, the mixture is extracted with 100 ml of ethyl acetate, the organic phase is dried with MgSO₄ and concentrated on a rotary evaporator in vacuo, and the residue is treated with tert-butyl methyl ether. The precipitate which forms is filtered off and discarded, and the solution which remains is concentrated to give 1.54 g (80%) of the mercapto compound of the formula VIIIa where R¹ =t-Bu, R³ =Me, R⁵ =Ph.

¹ H-NMR (200 MHz, CDCl₃): σ=0.92 (d; 3H, CHCH₃); 1.35 (s; 9H, C(CH₃)₃); 2.8-2.9 (m; 2H, CH₂ CH₂); 3.2-3.3 (m; 2H, CH₂ CH₂); 4.78 (quint; 1H, CHCHCH₃); 5.68 (d; 1H, CHCHPh); 7.25-7.50 (m; 5H, Ph). m.p.: 65°-67° C.

EXAMPLE 3 Synthesis of Compound VIIIa Where R¹ =C(CH₃)₃, R³ =Me, R⁵ =Ph

0.7 g (3.0 mmol) of the compound IVa where R³ =Me, R⁵ =Ph, 0.012 g (0.3 mmol) of NaOH in 0.5 ml of water, 10.5 mg of tetra-n-butylammonium hydrogen sulfate and 0.39 ml (3.4 mmol) of tert-BuSH are stirred, in a procedure similar to Example 2, in 4-5 ml of methylene chloride for 2 h at room temperature. Working up (extraction with CH₂ Cl₂, drying with MgSO₄) as in Example 2 gives 723 mg (75%) of the compound VIIIa where R³ =Me, R⁵ =Ph.

¹ H-NMR (200 MHz, CDCl₃): σ=0.92 (d; 3H, CHCH₃); 1.35 (s; 9H, C(CH₃)₃); 2.8-2.9 (m; 2H, CH₂ CH₂); 3.2-3.3 (m; 2H, CH₂ CH₂); 4.78 (quint; 1H, CHCHCH₃); 5.68 (d; 1H, CHCHPh); 7.25-7.50 (m; 5H, Ph). m.p.: 65°-68° C.

EXAMPLE 4 Synthesis of Compound VIIIb Where R¹ =C(CH₃)₃, R⁴ =Me, R⁶ =Ph

7.2 g (about 180 mmol) of NaH (55-60% pure) are placed in a 1 liter 4-necked flask with 200 ml of dry tetrahydrofuran. A mixture of 20 g (112.8 mmol) of the oxazolidinone of the formula IIb where R⁴ =Me, R⁶ =Ph and 80 ml of dry THF is added dropwise thereto at room temperature. The reaction mixture is stirred for one hour at room temperature and then cooled to from -60° to -65° C., and 11.8 ml (146.64 mmol) of acryloyl chloride are then slowly added. After stirring for 10 minutes the cooling bath is removed and 120 ml of water are slowly added. A pH of from 11 to 12 is then established with 5N NaOH. When the pH remains constant, 450 to 500 mg of methyltrioctylammonium chloride and 13.16 ml (116 mmol) of tert-butyl mercaptan are added. After from 30 to 40 minutes, about 400 ml of water and about 400 ml of ethyl acetate are added, the mixture is extracted by shaking, the organic phase is separated off, the aqueous phase is extracted again with 200 ml of ethyl acetate, and the combined organic phases are washed with saturated NaCl solution. After drying with Na₂ SO₄ or MgSO₄, the organic solution is concentrated by evaporation in vacuo and the residue is taken up in about 300 ml of tert-butyl methyl ether. Following filtration, the etherial solution is concentrated. The residue is brought to crystallization using n-heptane/ethyl acetate or n-heptane/tert-BuOMe. After complete crystallization the yield is 26.08-26.80 g (72-74%).

¹ H-NMR (200 MHz, CDCl₃): σ=0.92 (d; 3H, CHCH₃); 1.35 (s; 9H, C(CH₃)₃); 2.8-2.9 (m; 2H, CH₂ CH₂); 3.2-3.3 (m; 2H, CH₂ CH₂); 4.78 (quint; 1H, CHCHCH₃); 5.68 (d; 1H, CHCHPh); 7.25-7.50 (m; 5H, Ph). m.p.: 68°-70° C.

EXAMPLE 5 Synthesis of Compound VIIIa Where R¹ =C(CH₃)₃, R³ =Me, R⁵ =Ph

1.8 g (about 45 mmol) of NaH (60% pure) are washed with petroleum ether, and then 50 ml of THF are added. 5 g (28.2 mmol) of the oxazolidinone of the formula IIa where R³ =Me, R⁵ =Ph, are then dissolved in 20 ml of THF and this solution is added dropwise to the NaH/THF mixture. After stirring for one hour at room temperature the solution is cooled to -20° C., 2.35 ml (29.03 mmol) of acryloyl chloride are slowly added, and the mixture is stirred for a further 5 minutes.

18 ml of water are subsequently added, the mixture is warmed to room temperature, a pH of 10 is established, and 75 mg of tetra-n-butylammonium hydrogen sulfate are added and 2.87 ml (25.45 mmol) of tert-butyl mercaptan are added. After stirring for 2 hours at room temperature, 300 ml of water and 250 ml of ethyl acetate are added. Following extraction and drying with Na₂ SO₄ the organic solution is concentrated in vacuo. The residue is stirred together with tert-butyl methyl ether. The fine precipitate which forms is filtered off and the etherial solution is concentrated to dryness by evaporation. Filtration over 10-15 g of silica gel (in CH₂ Cl₂) and recrystallization from petroleum ether give 5.8-6.0 g (64-66%) of the pure Michael adduct of the formula VIIIa where R¹ =C(CH₃)₃, R³ =Me, R⁵ =Ph.

¹ H-NMR (200 MHz, CDCl₃): σ=0.92 (d; 3H, CHCH₃); 1.35 (s; 9H, C (CH₃)₃); 2.8-2.9 (m; 2H, CH₂ CH₂); 3.2-3.3 (m; 2H, CH₂ CH₂); 4.78 (quint; 1H, CHCHCH₃); 5.68 (d; 1H, CHCHPh); 7.25-7.50 (m; 5H, Ph). m.p.: 73°-75° C.

EXAMPLE 6 Synthesis of Compound VIIIb Where R¹ =C(CH₃)₃, R⁴ =Me, R⁶ =Ph

0.676 g (16.92 mmol) of NaH (60%) are placed together with 25 ml of THF (dry). A solution of 2.5 g (14.1 mmol) of the oxazolidinone of the formula IIb where R⁴ =Me, R⁶ =Ph, in 10 ml of THF (dry) is added dropwise thereto at room temperature. After stirring for 1 h at room temperature, the mixture is cooled to -60° C. and 1.19 ml (14.8 mmol) of acryloyl chloride are slowly added dropwise. After about 10 minutes the reaction has ended. A previously prepared mixture of 2.0 ml (18.33 mmol) of t-butyl mercaptan and 19.2 ml (21.15 mmol) of a 1.1 molar solution of tetra-n-butylammonium fluoride in THF is then added dropwise. After 2 h at -60° C., 150 ml of water and 150 ml of ethyl acetate are added, the mixture is extracted by shaking, the organic phase is dried with MgSO₄ and the solvent is removed in vacuo. The residue is dissolved in tert-butyl methyl ether, the fine precipitate formed is filtered off with suction over a clarifying layer and discarded, and the etherial phase is concentrated in vacuo. The residue slowly solidifies. Recrystallization or filtration over a little silica gel (in CH₂ Cl₂) gives 3.4 g (75%) of the desired Michael adduct of the formula VIIIb where R¹ =C(CH₃)₃.

¹ H-NMR (200 MHz, CDCl₃): σ=0.92 (d; 3H, CHCH₃); 1.35 (s; 9H, C (CH₃)₃); 2.8-2.9 (m; 2H, CH₂ CH₂); 3.2-3.3 (m; 2H, CH₂ CH₂); 4.78 (quint; 1H, CHCHCH₃); 5.68 (d; 1H, CHCHPh); 7.25-7.50 (m; 5H, Ph). m.p.: 69°-73° C.

EXAMPLE 7 Synthesis of Compound VIIIb Where R¹ =C(CH₃)₃, R⁴ =Me, R⁶ =Ph

2.82 g (70.5 mmol) of NaH (60% pure) are washed with petroleum ether. 50 ml of dried tetrahydrofuran are then added, and a solution of 5 g (28.2 mmol) of the oxazolidinone of the formula IIb where R⁴ =Me, R⁶ =Ph, in 20 ml of THF is then added dropwise with stirring at room temperature. After stirring for one hour at room temperature, the mixture is cooled to -60° C. and 2.35 ml (29 mmol) of acryloyl chloride are slowly added, and the mixture is stirred for a further 10 minutes. 3.49 ml (31 mmol) of tert-butyl mercaptan are then added thereto at -60° C. After about 1.0-1.5 hours the reaction mixture is slowly added to 100 ml of saturated ammonium chloride solution. After it has warmed to room temperature, 200 ml of water and 300 ml of ethyl acetate are added. The mixture is extracted by shaking, the organic phase is separated off and dried with MgSO₄, and the solution is concentrated in vacuo. Tert-butyl methyl ether is added to the residue which remains, the turbidity is removed by filtration and the clear etherial solution is subsequently concentrated by evaporation. The residue crystallizes under cold conditions. Filtration over a little silica gel and recrystallization from n-heptane give 5.89 g (65%) of the desired compound VIIIb where R¹ =C(CH₃)₃, R⁴ =Me, R⁶ =Ph.

¹ H-NMR (200 MHz, CDCl₃): σ=0.92 (d; 3H, CHCH₃); 1.35 (s; 9H, C(CH₃)₃); 2.8-2.9 (m; 2H, CH₂ CH₂); 3.2-3.3 (m; 2H, CH₂ CH₂); 4.78 (quint; 1H, CHCHCH₃); 5.68 (d; 1H, CHCHPh); 7.25-7.50 (m; 5H, Ph). m.p.: 73°-75° C.

EXAMPLE 8 Synthesis of Compound (S)-XII where R¹ =C(CH₃)₃ and R² =1-Naphthyl

0.59 ml (4.2 mmol) of diisopropylamine are placed in 7 ml of THF. 2.5 ml (4.04 mmol) of BuLi (1.6M in hexane) are added dropwise at -60° C. The mixture is stirred at -60° C. for 10 minutes and then a solution of 1 g (3.11 mmol) of the compound of the formula VIIIa where R¹ =tBu, R³ =Me, R⁵ =Ph in 3 ml of THF is added dropwise. Stirring is continued for 20 minutes and then a solution of 1.37 g (6.22 mmol) of 1-bromomethylnaphthalene in 3 ml of THF is added dropwise. The mixture is stirred for 5 minutes at -60° C. The temperature is subsequently allowed to rise to -10° C. over the course of 15 minutes by removing the cooling bath. After 40-60 minutes no more starting material is present, and 0.40 g (9.5 mmol) of LiOH.H₂ O and 1.0 ml (12 mmol) of H₂ O₂ (35% strength) in 1 ml of H₂ O are added. The mixture is stirred for about 30 minutes at room temperature, the THF is concentrated by evaporation in vacuo, and the remaining reaction mixture is extracted with CH₂ Cl₂. The aqueous phase is then acidified to pH=1 and is extracted thoroughly with ethyl acetate. The organic phase is dried with Na₂ SO₄ and concentrated. 594 mg (60%) of the desired product are obtained.

¹ H-NMR (200 MHz, d₆ -DMSO): σ=1.10, 1.15 (s; 9H, SC(CH₃)₃); 2.70-3.70 (m; 5H, CH and CH₂); 7.1-8.2 (m; 7H, aromat.); 12.6 (br; 1H, COOH).

EXAMPLE 9 Conversion of the Alkylation Product of the Formula Xb Where R⁴ =Me, R⁶ =Ph, R¹ =tBu into the (2R)-Sulfoxylcarboxylic Acid of the Formula XII Where R¹ =C(CH₃)₃ and R² =1-Naphthyl

200 mg (0.43 mmol) of the compound Xb where R⁴ =Me, R⁶ =Ph, R¹ =tBu, R² =1-naphthyl are dissolved in 2 ml of THF, and 0.11 ml (1.3 mmol) of H₂ O₂ (35% pure) and 55 mg (1.3 mmol) of LiOH.H₂ O in 1 ml of water are added, and the mixture is stirred at room temperature. The reaction has ended after 30-40 minutes. The THF phase is separated off and then the reaction mixture is extracted with CH₂ Cl₂ (recovery of the chiral auxiliary), and then the aqueous phase is acidified (pH=1) and the desired carboxylic acid is extracted with ethyl acetate. Drying (Na₂ SO₄) and concentration in vacuo give 126 mg (92%) of the desired (2R)-sulfonylcarboxylic acid XII.

MS (70 eV): m/z (%)=319.2 (100, M.sup.⊕ +1); 262.2 (25, M--C₄ H₈).

EXAMPLE 10 Oxidation of the (2S)-Sulfoxide of the Formula XII Where R¹ =C(CH₃)₃, R² =Naphthyl to Give the (2S)-Sulfone of the Formula I (R¹ =C(CH₃)₃, R² =naphthyl)

500 mg (1.57 mmol) of (S)-sulfoxide XII (where R¹ =C(CH₃)₃ and R² =naphthyl) are dissolved in 1 ml of CH₂ Cl₂.0.3 ml of glacial acetic acid, 0.52 ml of H₂ O₂ and 0.16 g of polyphosphoric acid are added in succession. The reaction has ended after stirring for 2 h. Water is added to the reaction mixture and the phases are separated. The organic phase is washed twice with water, dried (Na₂ SO₄) and concentrated on a rotary evaporator. 444 mg (84.7%) of the desired compound I in the (S) configuration are obtained.

¹ H-NMR (270 MHz, d₆ -DMSO) σ=1.25 (s; 9H, O₂ SC(CH₃)₃); 3.1-3.65 (m; 5H, CH₂ and CH); 7.35-8.2 (m; 7H, aromat.); 12.55 (br.; 1H, COOH). MS (70 eV):m/z (%)=334 (100, M.sup.⊕).

EXAMPLE 11 Oxidation of the (2S)-Sulfoxide of the Formula XII Where R¹ =C(CH₃)₃ to Give the (2S)-Sulfone of the Formula I (R¹ =C(CH₃)₃), R² =Phenyl)

790 mg (5 mmol) of KMnO₄ in 20 ml of water are added dropwise at room temperature to an aqueous solution (10 ml) of 1.34 g (5 mmol) of the mentioned (S)-sulfoxide formula XII and 300-500 mg of K₂ CO₃. After the reaction has ended (monitoring by TLC) precipitated manganese dioxide was filtered off, the aqueous solution is acidified and the desired sulfone is extracted with ethyl acetate. Drying (Na₂ SO₄) and concentration of the organic solution in vacuo give 1.25-1.35 g (88-95%) of the desired compound I where R¹ =t-Bu, and R² =phenyl, in the (S) configuration.

¹ HNMR (200 MHz, CDCl₃) σ=1.3 (s; 9H, O₂ S C(CH₃)₃); 2.9-3.6 (m; 5H, CH₂ n. CH); 7.15-7.35 (m; 5H, Ph). MS (70 eV): m/z=284 (M.sup.⊕)

EXAMPLE 12 Oxidation of the (2R)-Sulfoxide of the Formula XII Where R¹ =C(CH₃)₃, R² =Phenyl to Give the (2R)-Sulfone of the Formula I (R¹ =tBu, R² =Phenyl)

670 mg (2.5 mmol) of the (R)-sulfoxide of the formula XII are placed in 5-10 ml of t-butyl methyl ether and 2-5 ml of water, and 2.5 g of Oxone in 10 ml of water is added dropwise at 0° C. The mixture is subsequently stirred at room temperature. After the reaction has taken place (monitoring by TLC) and the reaction mixture has been acidified the phases are separated. The aqueous phase is extracted twice with ethyl acetate and the combined organic phases are dried with MgSO₄. 682 mg (96%) of the desired sulfone of the formula I where R¹ =C(CH₃)₃ and R² =phenyl are obtained in the (R) configuration.

¹ H-NMR (200 MHz, CDCl₃) σ=1.3 (s; 9H, O₂ S C (CH₃)₃); 2.9-3.6 (m; 5H, CH₂ n. CH); 7.15-7.35 (m; 5H, Ph). MS (70 eV): m/z=284 (M.sup.⊕)

EXAMPLE 13 Synthesis of (4S,5R)-3-(1-Oxo-3-chloropropyl)-4-methyl-5-phenyl-2-oxazolidinone (Compound of the Formula VIb Where R⁴ =Methyl, R⁶ =Phenyl, X=Cl)

0.52 g (12 mmol) of NaH (60%) are placed in 2 ml of THF. A solution of 1.32 g (7.45 mmol) of (4S,5R)-4-methyl-5-phenyl-2-oxazolidinone in 13 ml of THF is added dropwise thereto with stirring. After stirring for 30-40 minutes at room temperature, 1.23 g (9.7 mmol) of 3-chloropropionyl chloride are slowly added dropwise thereto at -60° C. and the mixture is then stirred for about 30 min. at -60° C. 10 to 15 ml of citric acid/water are then added at from -60° to -40° C., the reaction temperature is allowed to rise slowly to 25° C., the reaction mixture is extracted by shaking with ethyl acetate, and the organic phase is dried with MgSO₄ and concentrated in vacuo. Crystallization gives 1.3-1.4 g (65-70%) of the desired product.

¹ H-NMR (200 MHz, CDCl₃): σ=0.92 (d; 1H, CHCH₃); 3.4-3.5 (m; 2H, CH₂ CH₂); 3.8-3.9 (m; 2H, CH₂ CH₂); 4.80 (quint; 1H, CHCHCH₃); 5.70 (d; 1H, CHCHPh); 7.2-7.5 (m; 5H, Ph).

EXAMPLE 14 Precipitation of Compounds of the Formula I As Cyclohexylammonium Salt

21 g of I are dissolved in 200 ml of acetone, and 6 g of cyclohexylamine are added. After dilution with 100 ml of acetone the solid product is filtered off with suction. 20.4 g of the cyclohexylammonium salt of I are obtained.

EXAMPLE 15 Liberation of Compounds of the Formula I from the Cyclohexylammonium Salt

15 g of the salt are admixed with 100 ml of ethyl acetate and 100 ml of 2N HCl, and the mixture is extracted by shaking. The organic phase is washed with 100 ml of saturated NaCl solution, dried with Na₂ SO₄ and concentrated. 12 g of the free acid I are obtained.

EXAMPLE 16 Differentiation of (S)-I and (R)-I

(S)-I and/or (R)-I, dissolved in methylene chloride, are cooled to 0° C., and an excess of triethylamine, (S)-phenylalaninol and propanephosphonic anhydride is added, and then the mixture is stirred for about 1 h at room temperature. The diastereomeric amides can be differentiated both by HPLC and by TLC: e.g. for I where R¹ =tBu, R² =naphthyl:

    (S)-I-(S)-phenylalaninolamide Rf (tBuOMe) 0.60

    (R)-I-(S)-phenylalaninolamide Rf (tBuOMe) 0.32

EXAMPLE 17 Synthesis of Compound VIIIa Where R¹ =C(CH₃)₃, R³ =Me, R⁵ =Ph

2 ml of abs. THF are added to 0.515 g (11.8 mmol) of NaH (about 60%). Then 1.32 g (7.45 mmol) of the oxazolidinone IIa where R³ =Me, R⁵ =Ph are added and the mixture is stirred for 0.5-1.0 h at room temperature. It is cooled to -60° C. and then 0.72 ml (7.45 mmol) of 3-chloropropionyl chloride is slowly added.

The reaction mixture is stirred for 30-45 min. at -60° C., and then a mixture of 20 mg of tetrabutylammonium hydrogen sulfate, 15 ml of water and 0.86 ml (7.66 mmol) of tert-butyl mercaptan is added. The reaction mixture is allowed to warm up slowly to room temperature, a pH of 9-11 is established and the mixture is stirred for 1 to 2 hours at room temperature.

Subsequently, following the addition of from 50 to 100 ml of water, the mixture is extracted with ethyl acetate, and the organic phase is dried and concentrated; crude product: 2.3 g.

Filtration over about 6 g of silica gel and crystallization from cyclohexane give 1.7 g (73%) of the desired compound VIII where R¹ =C(CH₃)₃.

Spectroscopic data: ¹ H-NMR (200 MHz, CDCl₃): σ=0.92 (d; 3H, CHCH₃); 1.35 (s; 9H, C (CH₃)₃); 2.8-2.9 (m; 2H, CH₂ CH₂); 3.2-3.3 (m; 2H, CH₂ CH₂); 4.78 (quint; 1H, CHCHCH₃); 5.68 (d; 1H, CHCHPh); 7.25-7.50 (m; 5H, Ph).

EXAMPLE 18 Synthesis of Compound VIIIa Where R¹ =C(CH₃)₃, R³ =Me, R⁵ =Ph

2.6 g (59 mmol) of NaH (60% dispersion) and 6.6 g (37.25 mmol) of the oxazolidinone of the formula IIa where R³ =Me, R⁵ =Ph are stirred in 10 ml of abs. THF for one hour at room temperature. The mixture is then cooled to -60° C., 3.58 ml (37.25 mmol) of chloropropionyl chloride are added dropwise, and the mixture is stirred for 30-45 min. at -60° C. Following the addition of 1.3 g (6.8 mmol) of dry citric acid, the mixture is stirred for 5-15 min at 60° C., then 50 ml of water are slowly added, the temperature is raised to from -10° to 0° C., 100 mg of nBu₄ NHSO₄ are added, and a pH of 9-10 is established with 5N NaOH. Following the addition of 4.31 ml (38.6 mmol) of tert-butyl mercaptan, the mixture is stirred for 2 h at room temperature. In order to work up the reaction, 200 ml of ethyl acetate and 200 ml of semisaturated citric acid solution are added, the mixture is extracted with shaking, and the organic phase is washed with water, dried with MgSO₄ and concentrated in vacuo;

crude yield 12 g. Filtration over about 25 g of silica gel (CH₂ Cl₂) and extraction by stirring with hexane or pentane give 7.8 g (65%) of the desired product.

m.p.: 73°-75° C. 

We claim:
 1. A process for the stereoselective preparation of a compound of the formula I ##STR10## in which R¹ is C₁ -C₆ -alkyl, C₃ -C₆ -cycloalkyl, (C₆ -C₁₂)-aryl-(C₁ -C₄)-alkyl or C₆ -C₁₂ -aryl or -heteroaryl or -heterocycloalkyl which is unsubstituted or substituted by 1, 2 or 3 identical or different hydroxyl, methoxy or trialkylsilyloxy groups; andR² is C₆ -C₁₂ -aryl which is unsubstituted or substituted by 1, 2 or 3 identical or different methoxy, halogen, cyano, methyl, trifluoromethyl, isopropyl or nitro groups; is C₃ -C₉ -heteroaryl which is unsubstituted or substituted by 1, 2 or 3 identical or different methoxy, halogen, cyano, methyl, trifluoromethyl, isopropyl or nitro groups; or is C₁ -C₁₀ -alkyl, alkenyl or alkynyl which is unsubstituted or substituted by 1, 2 or 3 identical or different methoxy, halogen, cyano, methyl, trifluoromethyl, isopropyl or nitro groups and the compounds of the formula I may be present in the R or S form, which process comprises reacting a compound of the formula IIa or IIb or IIIa or IIIb, respectively, (chiral auxiliary), in which Y is =O or S, ##STR11##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.5 =C.sub.6 -C.sub.10 -aryl, R.sup.4 =R.sup.6 =H,                                              IIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.10 -aryl, R.sup.3 =R.sup.5 =H,                                              IIb

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      IIIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,

a₁) with acrylic acid or with a derivative of acrylic acid, to form a compound of the formula IVa or IVb or Va or Vb, respectively, in which Y is =O or S, ##STR12##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.5 =C.sub.6 -C.sub.10 -aryl, R.sup.4 =R.sup.6 =H,                                              IVa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.10 -aryl, R.sup.3 =R.sup.5 =H,                                              IVb

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      Va

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,                      Vb

or a₂) with propionic acid or a propionic acid derivative which are in turn substituted in position C₃ by a radical X=Cl, Br, OTs, OMs, to form a compound of the formula VIa or VIb or VIIa or VIIb, respectively, in which X and Y are as defined above, ##STR13##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.5 =C.sub.6 -C.sub.10 -aryl, R.sup.4 =R.sup.6 =H,                                              VIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.10 -aryl, R.sup.3 =R.sup.5 =H,                                              VIb

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      VIIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,                      VIIb

b) reacting the compound of the formula IVa, IVb, Va, Vb, VIa, VIb, VIIa or VIIb obtained according to a₁) or a₂), directly or after isolation in a nonaqueous medium or in the presence of a phase-transfer catalyst in an aqueous two-phase system, with a mercaptan of the formula R¹ SH, in which R¹ is as defined above, into the corresponding compound of the formula VIIIa or VIIIb or IXa or IXb, respectively, in which R¹ and Y are as defined above, ##STR14##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.5 =C.sub.6 -C.sub.10 -aryl, R.sup.4 =R.sup.6 =H,                                              VIIIa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.10 -aryl, R.sup.3 =R.sup.5 =H,                                              VIIIb

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      IXa

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,                      IXb

or a₃) converting a compound of the formula IIa or IIb or IIIa or IIIb, respectively, in the presence of a quantity of a base which is sufficient for the deprotonation of the oxazolidinone and of the mercaptan R¹ SH, in which R¹ is as defined above, into a compound of the formula VIIIa or VIIIb or IXa or IXb, respectively, by reaction with acryloyl or propionyl halides or their respective mixed anhydrides and further reaction of the reaction product with the mercaptan in one step, c) then converting the compound obtained according to b) or a₃) into the corresponding compound of the formula Xa, Xb, XIa or XIb by stereoselective alkylation, ##STR15##

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, R.sup.4 =R.sup.6 =H, R.sup.5 =C.sub.6 -C.sub.12 -aryl,                                          Xa

    R.sup.3 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.4 =R.sup.5 =R.sup.6 =H,                      XIa ##STR16##

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, R.sup.6 =C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =H,                                              Xb

    R.sup.4 =C.sub.1 -C.sub.4 -alkyl, C.sub.1 -C.sub.2 -alkyl-C.sub.6 -C.sub.12 -aryl, R.sup.3 =R.sup.5 =R.sup.6 =H,                      XIb

in which Y, R¹ and R² are as defined above, d) then converting the compound obtained in this way by reaction with H₂ O₂ in alkaline solution, into compounds of the formula XII which may be present in the R or S form, ##STR17## and e) then converting the compound obtained in this way, by further oxidation, into the corresponding compound of the formula I.
 2. The process as claimed in claim 1, whereinR¹ is C₁ -C₄ -alkyl, C₅ -C₆ -cycloalkyl, (C₆ -C₁₂)-aryl-(C₁ -C₃)-alkyl or C₆ -C₁₂ -aryl or -heteroaryl which is unsubstituted or substituted by a hydroxyl, methoxy or trialkylsilyloxy group; and R² is C₆ -C₁₂ -aryl which is unsubstituted or substituted by a methoxy, halogen, methyl, trifluoromethyl or isopropyl group; is C₃ -C₆ -heteroaryl which is unsubstituted or substituted by a methoxy, halogen, methyl, trifluoromethyl or isopropyl group, or is C₁ -C₄ -alkyl, -alkenyl or -alkynyl.
 3. The process as claimed in claims 1 or 2, whereinR¹ is C₁ -C₄ -alkyl, (C₆ -C₁₂)-aryl-(C₁ -C₃)-alkyl or C₆ -C₁₂ -aryl, and R² is =C₆ -C₁₂ -aryl, C₃ -C₆ -heteroaryl or C₁ -C₄ -alkyl, -alkenyl or -alkynyl.
 4. The process as claimed in claim 1, wherein a compound of the formula IIa or IIb or IIIa or IIIb, respectively, in which

    R.sup.3 =Me, R.sup.5 =Ph, R.sup.4 =R.sup.6 =H              IIa

    R.sup.4 =Me, R.sup.6 =Ph, R.sup.3 =R.sup.5 =H              IIb

    R.sup.3 =CH(CH.sub.3).sub.2, R.sup.4 =R.sup.5 =R.sup.6 =H  IIIa

    R.sup.4 =CH(CH.sub.3).sub.2, R.sup.3 =R.sup.5 =R.sup.6 =H  IIIb

and Y is =O is employed.
 5. The process as claimed in claim 1, wherein the compound of the formula IIa or IIb or IIIa or IIIb, respectively, is converted in accordance with a₃) in one step directly into a compound of the formula VIIIa or VIIIb or IXa or IXb.
 6. The process as claimed in claim 1, wherein the alkylation of the compound of the formula VIIIa, VIIIb, IXa or IXb, the subsequent elimination of the chiral auxiliary and the oxidation to give the compounds of the formula XII or I, respectively, is carried out without isolating the intermediates.
 7. The process as claimed in claim 1, wherein in step (d) the compound is reacted with LiOH/H₂ O₂. 